This invention relates to Loran receivers. More particularly, the instant invention relates to apparatus for artificially simulating Loran navigation signals.
Loran is an abbreviation for "long-range navigation." A plurality of Loran radio stations is utilized for generating and transmitting pulse-modulated signals for reception by vessels wishing to electrically determine their geographical position. Loran-A systems operate on radio frequencies between 1800 and 2000 kilohertz, and transmit pulse-modulated radio waves. Loran navigation involves a master station and usually two slave stations which transmit time-delayed slave pulses in response to the master station.
To determine one's position through the use of Loran, hyperbolic lines of position are determined and/or plotted. The time difference between the master signal pulse and the subsequent slave signal pulse is determined, and a locus of points on a map may be plotted corresponding to that given time differential. The plot, which will be in the form of a hyperbola, indicates the locus of geographical points at which a given time difference between received master and slave pulses will be perceived. Subsequently, a second locus is plotted by computing the time differential between the master signal and a second slave signal. The intersection of the two hyperbolic loci thus generated will indicate the position of the vessel. Pulse positional navigational systems of this type are well known in the art and one example of such a system is shown in U.S. Pat. No. 2,855,594, issued to F. G. Bac on Oct. 7, 1958.
Sophisticated modern Loran or pulse-positional navigation receivers may comprise one or more R.F. input stages, a plurality of intermediate frequency amplifier stages, a detector stage and for eliciting time base information from the pulse modulation received thereby, and some sort of a display apparatus for visually aiding positional determination by reading out the derived time delay, for example. In modern Loran receivers the display apparatus comprises a cathode ray tube which is adapted to display the master and slave pulse signals and to aid the operator in synchronizing the receiver demodulation circuitry thereby determining the time differences between pulses necessary for plotting geographical position. Usually logic circuitry provided within the receiver will "decode" the pulse timing information and determine the time difference between received master and slave signals. Transistorized ring counters, for example, may be employed. The receiver operator must appropriately position the pulse signals on the diplay tube of the receiver. The display apparatus enables the operator to visually align the master and slave signal traces by adjustment to the receiver such that the pulse-detection network associated with the receiver will determine the time difference between pulses and thereby compute the proper hyperbolic navigational locus. When two or more locus lines are plotted in this manner, the intersection of the loci will indicate the position of the vessel.
Loran master signals are transmitted in the form of several "basic" pulse rates: H, L, S, SS, SH, SL, and SC. The most common rate, SS, involves the transmission of a master pulse every 100,000 microseconds, or 10 such pulses per second. The SC rate, for example, involves 20 master pulses per second. The L rate generates 25 master pulses per second. Loran systems also involve a "specific" rate which in effect yields more "range" to the measurement system. The specific rates run from SS 0 - SS 7. When a basic rate of SS is selected with a specific rate of SS 1, for example, master pulses are transmitted every 100,000-100 microseconds, or every 99,900 microseconds. Depending upon the pulse transmission rate of the received Loran station, the receiver must be adjusted to the appropriate "rates" to enable detection of the master-slave pulse delay differential.
In recent years, the price of Loran receivers has come down because of the advent of solid state design and construction techniques. Many modern salt water fishing and/or pleasure boats are now routinely equipped with Loran navigational aids. The Coastal areas of the United States are provided with a plurality of Loran transmitting stations so that reliable navigation through these areas is insured. As a result of the increasingly popular use of Loran receivers, there exists a definite need for a low-cost, easily operable portable Loran testing device which can be used for signal tracing, diagnostic testing and troubleshooting or the like. Unfortunately many of the prior art Loran testing systems are large, bulky heavy units which are unsuitable for portable operation. The bulkiness of some of these prior art units is at least in part due to their inclusion of an R.F. oscillator stage for generating an R.F. carrier signal between 1600 and 1800 kilocycles suitable for actuating typical Loran receivers. Also, pulse modulation circuitry is typically included to properly modulate the R.F. carrier.